On Support Weapons and Heavy weapons, it really depends on your intended role for the Guardian formation. A full set of 'shooters' (6x units) kicks out a lot of firepower that can severely dent an assaulting formation, so this makes it an ideal BTS guard on OW. Put the shooters in front of the other Guardians as a screen against enemy CC units and the formation will provide approximately the same stats in FF or CC (note this is also the best way to defend the WraithLords mentioned earlier). As always, positioning is crucial to minimse the threat of enemy firepower while maximising the potential of the formation. Also it is usually wise to have two or even three such BTS guards; Void Spinners, Fire Prisms and even Rangers are ideal companions.

In an assault role, having 'shooters' is much less valuable, but I sometimes change one or two Guardians into Heavy Weapons to allow  them to be moved into a supporting position while laying a BM by shooting.

But don't forget the way the upgrades are currently worded. Essentially you may either take the various infantry upgrades or you may add Wave Serpents to the unaltered Guardian formation.

Finally if you are intent on using an 'infantry' based army, try to play to it's strengths, which is large numbers of activations. Referring back to the 'pop-corn' idea below, instead of 9x Guardians (for 18 formations), you can always convert some of these into WraithGuard upgrades to give you a few very resilient assaulting formations eg
500 - 2x Void Spinners,
250 - 1x Falcons (+ Fire Storms),
300 - 1x Night Wings,
300 - 3x Rangers(4)
200 - 1x War Walkers
600 - 2x Guardians + WraithGuards
350 - 1x Guardians + WraithGuards + Support platforms (BTS)
450 - 3x Guardians
50  - Wraith Gate

A total of 14 activations

_________________
"Play up and play the game"

Vitai lampada
Sir Hemry Newbolt